Magnetic random access memory, or MRAM, is a burgeoning new technology that will provide non-volatile data storage in a compact, low-power device. MRAM comprises an array of cells where each cell stores one bit in at least one magnetoresistive (MR) element. Bits are defined by the magnetic configuration of a free ferromagnetic layer (FL) within an MR element. The magnetic orientation of the FL with respect to a pinned ferromagnetic layer (PL) is measured via a magnetoresistive effect during read-back.
MRAM technology is still in its infancy and has yet to reach the commercial market. Among the many challenges facing MRAM is the stabilization of a uniformly magnetized FL. Since common MRAM designs use magnetic fields generated by current carrying wires to write bits, the FL necessarily must have a net magnetic moment so that the magnetic write field can align the FL into its intended direction. However, the stabilization of a finite moment FL into a single domain state presents a challenge because of the need to combat energetically prohibitive free magnetic charges at the surface of the FL. There is a strong preference for the FL magnetization to achieve flux closure internally in the form of a vortex rather than having field lines close through free space. Even when a stable single domain state is achieved, the magnetostatic fields generated by the FL will interact with neighboring magnetoresistive elements, thereby limiting the packing density of cells. Flux closure is therefore a significant issue affecting the reproducibility of single domain bits, but a finite moment FL is an unavoidable obstacle for those MRAM designs that write bits with magnetic fields. Proposals to try to minimize this problem have called for the fabrication of a FL from a synthetic antiferromagnet (SAF), where two ferromagnetic layers are strongly coupled antiferromagnetically through a non-magnetic spacer layer. But, these proposals still require a net moment for the FL and are only a stopgap solution to the problem.
An alternative write mechanism is spin momentum transfer. This effect exploits the torque exerted by a spin polarized current of conduction electrons on the localized moments in a ferromagnet. No magnetic fields are required to write bits. The only requirements are that the magnetoresistive elements each contain a PL and FL and that electrical current can be passed through each device in a direction perpendicular to the planes (CPP) of the thin film multilayer memory element, which is also called a pillar. The FL can then be oriented into either parallel or antiparallel configurations with respect to the PL simply by applying a CPP current of sufficient magnitude and proper direction through the device. Spin transfer torque is significantly large for CPP pillars having cross-sectional areas on the order to 104 nm2 or less, and becomes more efficient with respect to write current with decreasing pillar size. Hence, there are proposals to use this effect in ultra-high density MRAM.
The concept of using a SAF structure to provide flux closure has been described in existing patents and has been proposed for use as a read sensor or in an MRAM cell. However, these proposals provide only a partial solution because the proposed SAF structures require a net moment in order to work properly in the magnetic fields of the intended application.
It would be desirable to provide an MRAM structure that uses a spin transfer write mechanism and includes flux closure for the free layer.